1. Field of the Invention
The present invention relates generally to a bias circuit, and more particularly to a bias circuit for a differential amplifier of the type of a single drive.
2. The Prior Art
In some differential amplifiers having single-ended input circuits, the base bias voltage is derived from an emitter-follower connected to the bases of both of the differentially connected transistors. Although the base of the differentially connected transistor to which the input signal is not applied could be connected to a low impedance supply circuit, the other differentially connected transistor, to which the input signal is applied, must have a relatively high impedance base circuit so as not to attenuate the input signal. The relatively high impedance circuit requires that there be a resistor between the base of the signal input transistor and the emitter of the emitter-follower circuit that supplies the base bias voltage. Base current flowing through that resistor affects the bias voltage at the base.
Balanced operation of the two differentially connected transistors requires that the other of the two transistors also have a resistor between its base and the emitter of the emitter-follower circuit. In order to hold the base of the second differentially connected transistor substantially at ground level, a capacitor may be connected between that base and ground. However, this is not satisfactory for integrated circuit use because of the necessity to provide a connection for the capacitor. Furthermore, it affects the low frequency response of the differential amplifier.
Balancing resistors may be inserted in the emitter leads of the differentially connected transistors instead of connecting the base of the second transistor to ground through a capacitor. However, these balancing resistors reduce the gain that may be obtained from the differential amplifier.